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#include <bits/stdc++.h>
using namespace std;

#define int long long

main()
{
ios_base::sync_with_stdio(false);
int n,k; cin >> n >> k;
vector<int> v;
for(int i=0; i<n; i++)
{
    int a; cin >> a; v.push_back(a);
}

unordered_set<int> used;
int sum = 0;
for(int i=0; i<n; i++)
{
    if(used.count(v[i])) continue;
    used.insert(v[i]);
    sum += i + 1;

    if(used.size() == k) break;
}

if(used.size() != k)
{
    std::cout << -1 << std::endl;
    return 0;
}
int base_sum = k*(k+1)/2;
std::cout << sum - base_sum << std::endl;
return 0;
}